(You may cast either half. That door unlocks on the battlefield. As a sorcery, you may pay the mana cost of a locked door to unlock it.) At the beginning of your upkeep, exile the top card of your library. You may play that card this turn.

(You may cast either half. That door unlocks on the battlefield. As a sorcery, you may pay the mana cost of a locked door to unlock it.) Once each turn, you may pay {0} rather than pay the mana cost for a spell you cast from exile.